Children of all ages are curious about why some objects float while others sink. They have probably had a number of experiences playing in water with a variety of objects. They may also have seen air bubbles rising through the water after someone jumps in, or watched boats, leaves, or sticks floating in a pond. Some may have noticed that they sink in the bathtub but float in the ocean.

From such observations and experiences, children have developed ideas and questions about floating and sinking. They may think that weight has something to do with why things sink or float. Others may suspect that size or even the liquid plays a role. How these factors and others affect the behavior of objects in liquids is the focus of Floating and Sinking, a 16-lesson unit for fifth-graders. The activities in this unit provide experiences upon which children can build a more complete understanding of the phenomenon of buoyancy.

Lesson 1 begins with a pre-unit assessment of students' ideas and questions about floating and sinking. In Lessons 2 through 5, they begin to test their ideas, focusing first on weight and size. In addition, they investigate and calibrate a device for weighing objects--a spring scale. Students make predictions and then test them with experiments. They record observations, discuss their ideas with classmates, measure the weight of a variety of objects, and begin a class graph of their observations. By the end of Lesson 5, students will have made surprising discoveries: some "floaters" are heavier than some "sinkers," and the largest objects are not always the heaviest.

In Lessons 6, 7, and 8, students test variables by constructing and testing boats of various shapes, sizes, and materials. Students make clay boats and explore how weight can be distributed to make a sinker into a floater. They then load them with marbles to see how much cargo different-sized boats will keep afloat. These activities give students information about the relationship between the variables of size, weight, and design.

In Lessons 9 through 12, students focus on the behavior of objects when they are submerged in water. First, they use the spring scale to measure the buoyant force on different-sized fishing bobbers. Then they measure the change in water level caused by objects submerged in a graduated tube.

When students measure the weight of a variety of submerged objects, they discover that because of the buoyant force, objects appear to weigh less when they are submerged. Holding size and volume constant, students weigh water and compare its weight with that of solid objects. This experience provides an interesting observation: objects that weigh more than the same volume of water sink, while objects that weigh less than the same volume of water float.

In Lessons 13, 14, and 15, students investigate the behavior of objects in salt water. They dissolve salt in water and compare the weight of the water with the weight of the salt water. Then they test the objects they have worked with previously to see if they float or sink in salt water. Here, students discover that because salt water is heavier than fresh water, some objects (the acrylic cylinders and the nylon bolt) that sank in fresh water can float in salt water. In Lesson 15, students construct a scientific instrument called a hydrometer and compare the level at which it floats in graduated cylinders of fresh and salt water. Through this activity, students expand their understanding of buoyant force and displacement.

Lesson 16 provides an excellent opportunity to assess student learning. Students are challenged to apply what they have learned from their previous experiences with buoyancy. They make and test predictions about an unknown "mystery cylinder."

In Floating and Sinking, students become engrossed in the activities and generate many questions about what they observe. To assist you in helping students find out answers for themselves, the unit provides background information, diagrams, extensions, and reading selections. Assessments that are embedded in many lessons give you information for evaluating students' learning. Matched pre- and post-unit assessments enable you to document growth in students' concepts of buoyancy. Additional assessments are also included in the unit, as well as a bibliography of other resources related to this topic.